In various electronic apparatuses such as mobile telephones and car navigation systems, such uses are prevalent in which, having a transparent touch panel (hereinafter referred to as TTP) mounted in front of a display device such as a liquid crystal display device, visual recognition or selection, through the TTP, of characters and symbols displayed on the display device behind the TTP, is made and switching operation between various functions of the apparatuses is made with a push given onto the TTP with a finger, dedicated pen, or the like.
Such a TTP will be described with reference to FIGS. 4A and 4B. Incidentally, the drawings are given in expanded dimension in the thickness direction of the TTP for ease of understanding the configuration of the TTP.
FIG. 4A is a sectional view of a conventional TTP. Upper substrate 1 is made into a transparent film form. Lower substrate 3 is also made transparent. On the lower face of upper substrate 1, there is formed upper conductive layer 2 made of such a transparent material as indium tin oxide (ITO). Also on the upper face of lower substrate 3, there is formed a transparent lower conductive layer 4.
Between upper conductive layer 2 and lower conductive layer 4, there is sandwiched pressure sensitive conductive sheet (hereinafter referred to as PSCS) 5, which is transparent and of such a configuration that has a plurality of opaque conductive particles 7, having an average particle size of 50 μm, dispersed in elastic member 6 of transparent rubber or the like.
There are disposed a pair of upper electrodes (not shown) at both ends in one direction of upper conductive layer 2. Further, at both ends in the other direction of lower conductive layer 4, there are disposed a pair of lower electrodes (not shown) perpendicularly to the upper electrodes, and thus a TTP is constructed.
Such a TTP configured as described above is mounted on an electronic apparatus as disposed in front of a display device of a liquid crystal or the like, while the pairs of upper electrodes and lower electrodes are connected to electronic circuits (not shown) of the electronic apparatus.
In the above described configuration, if a predetermined position in the upper surface of upper substrate 1 is given a push with a finger, pen, or the like while a display on the display device behind the TTP is identified by sight, then upper substrate 1 is bent as shown in FIG. 4B, so that PSCS 5 is pressed down at the pushed position and the plurality of conductive particles 7 at the pushed position are brought closer to each other to come into contact with each other.
If a voltage is applied from the electronic circuit to the upper electrodes and the lower electrodes serially, the span between upper conductive layer 2 and lower conductive layer 4 at the pushed position conducts a current through the mutually contacted plurality of conductive particles 7. According to the ratios of voltages between these electrodes, the pushed position is detected by the electronic circuit and, thereby, various functions of the electronic apparatus are switched over. An example of a TTP configured as described above is disclosed in Japanese Patent Unexamined Publication No. H5-143219.
However, in the above TTP of a related art example, the average particle size of the opaque conductive particles 7 within elastic member 6 is as large as 50 μm. Therefore, when the added amount of conductive particles 7 is increased, the transparency of the pressure sensitive conductive sheet 5 is impaired and, hence, it becomes difficult to visually recognize the display on the display device behind the TTP. When, in reverse, the added amount of conductive particles 7 is decreased, the distances between the dispersed conductive particles 7 become larger leading to unstable contacts between conductive particles 7 when a push operation is made. There has been involved such a problem in the related art.